The present invention relates to a calcium-nickel base alloy that stores large amounts of hydrogen at a predetermined temperature and hydrogen gas pressure and which is capable of readily releasing hydrogen either by slight pressurization or by reduction of hydrogen gas pressure or by a combination of these methods.
Hydrogen is expected to become the core element of the secondary forms of energy to be employed in the projected future. While hydrogen may be stored and transmitted in the form of a high-pressure gas or liquid hydrogen or even as a solid metal hydride, the last mentioned form is gaining increasing interest in the laboratory due to safety considerations and the ease of handling.
The following reasons may be given to explain the potential utility of metal hydrides: (1) they can store at least 1,000 times as much hydrogen per unit volume as hydrogen gas containers and can store as much hydrogen gas as liquid hydrogen vessels; (2) metal hydrides do not require the use of pressure vessels for hydrogen storage and no special caution need be exercized with respect to pressure resistance or hydrogen embrittlement; (3) metal hydrides are thermodynamically stable and permit extended hydrogen storage without experiencing the considerable hydrogen loss that occurs in the case of liquid hydrogen; and (4) most metal hydrides have a generally constant dissociation pressure so that when heated to a dissociation temperature, hydrogen gas of a constant pressure is generated.
In addition to storage containers, the potential use of metal hydrides covers fuel cells, fuel tanks for internal combustion engines, hydrogen refiners, home heating and cooling, compressors and refrigerators. Metal hydrides have many advantages over other conventional means for hydrogen storage and transmission in terms of improved safety, availability of simpler equipment, and enhanced performance.
While immobilization of hydrogen in the form of metal hydrides holds great promise as an elegant method of hydrogen storage and transmission, commercial alloys for hydrogen storage must meet the following requirements: (1) they are easily activated, (2) they suffer minimum deterioration as a result of cyclic hydrogen occlusion and release, (3) pressure equilibrium for the formation and dissociation of metal hydrides can be freely selected over a broad temperature range, (5) pressure equilibrium curves for hydrogen occlusion and release have minimum hysteresis, (6) rapidity of hydrogen occlusion and release, and (7) low cost.
While various alloys for hydrogen storage have been proposed to date, one that holds the most promise for commercialization is that disclosed in Japanese Patent Application No. 58-60075, namely: EQU CaNi.sub.5-(x+y) Mm.sub.x Al.sub.y,
where Mm is a misch metal, x and y are respectively atom ratios of Mm and Al with Ca taken as unity, and 0&lt;x.ltoreq.0.4 and 0&lt;y.ltoreq.0.6. Most conventional CaNi.sub.5 base binary alloys are fairly thermally unstable, and as hydrogen occlusion and release cycles are repeated, an alloy phase that is inactive to hydrogen is irreversibly precipitated from the CaNi.sub.5, causing a drop in the alloy's ability to occlude hydrogen. The proposed alloy suppresses this problem by the combined addition of a misch metal and aluminum.
The present inventors made continued studies on this alloy and found the following:
(1) Since nickel is partly replaced by a misch metal and aluminum, the composition of the alloy is determined by the amounts of misch metal and aluminum and it is difficult to allow for free selection of the equilibrium pressures of a metal hydride formation and dissociation in the neighborhood of room temperature; and (2) the reduced nickel content leads to a smaller proportion of the CaNi.sub.5 responsible for hydrogen occlusion, and this unavoidably results in a decreased hydrogen occlusion by the alloy.